Some whiteware appliances, such as horizontal axis washing machines, have a wash drum that rotates around a central horizontal axis. The drum can be directly rotated by a motor fixed to the rear wall of the drum. Alternatively, the wash drum can be rotated by a motor located proximate the drum and coupled to it via an appropriate drive mechanism, for example, a belt and pulley transmission.
One type of motor now used in the whiteware industry is an electronically commutated brushless external rotor motor. Electronically commutated motors are described in further detail in U.S. Pat. No. 5,821,708, the contents of which are hereby incorporated by reference. Such electronically commutated motors are designed to typically operate at speeds of up to 1000 rpm or more.
Low speed rotation of the drum is required in some machines described in the prior art to enable tasks such as washing machine door opening or closing procedures. For example, a door opening procedure requiring low speed rotation is described in U.S. Pat. No. 7,065,905. Low speed rotation may also be used to allow fluid to slowly drain from a wash drum as it rotates. Such fluid could be residual wash fluid, or fluid that has been captive within balancing chambers during wash cycles. Low speed rotation, however, has inherent difficulties.
One such difficulty of low speed rotation of the motor is increased risk of the motor stalling. Stalling is undesirable due to the large amounts of electrical energy required to reinitiate rotation. Stalling may occur due to an unbalanced wash load distributed within the wash drum. As the wash load is lifted by the drum, the increasing weight may cause the motor to decelerate and stall.
Similarly, another difficulty arises as the wash load begins to travel downward. The motor may accelerate undesirably. The acceleration may cause, for example, heavy collisions at the end of a door opening procedure.